Transmissions with a large range of narrow gear ratios are necessary, especially for use in commercial vehicles such as trucks or busses. In order to achieve the high number of gears with reasonable costs, as well as with as low weight and construction space requirements as possible, frequently a so-called range group transmission is used, which is formed from a multi-speed main transmission and a usually two-stage range group arranged downstream in relation to this.
The main transmission itself has, in this connection, a relatively low range from 2 to 3 and is usually constructed as a spur gear transmission in a gearbox design. The shifting clutches of the main transmission can be realized synchronized or unsynchronized, i.e., as friction ring synchronous clutches or as claw clutches. The shifting, thus changing the gear of the main transmission, can basically take place manually or automatically. Nonetheless, presently the discussion proceeds from automatic shifting of the main transmission for which the latter is provided with a transmission brake in connection with the affected input shaft, as it is known from German Patent DE 102 10 177 A1.
In contrast, the range group has a relatively large spread in the range from about 2.5 to 5. A spread of the overall transmission from 5 to 15 results from this and, in the event of four gears of the main transmission as well as two gears of the range group (an L-reduction gear for a slow gear group and an S gear for a fast gear group), a total of 8 gears result. The small or slow gears of the overall transmission are formed by the gears of the main transmission when the L gear of the range group is engaged and the large or fast gear groups by the same gears of the main transmission when the S gear of the range group is engaged.
The range group is, for example, constructed as a simple planetary gearset. The sun gear is connected with the input shaft and its planet carriers with the output shaft of the overall transmission. With a range group constructed in this manner, the L gear is shifted by arresting the ring gear on the transmission housing and the S group by a coupling of the ring gear with the planet carrier. Alternatively, the range group can, however, also be constructed as a spur gear transmission in gearbox design.
In the event of a range change shift, a change of the gear is conducted, in each case, in the main transmission, as well as in the downstream range group. In the event of a range change-up shift, a shift is made in the range group from the L gear into the S gear, while in the range transmission, there is a shift back from the greatest gear, usually constructed in the form of a direct gear, into the smallest gear. Conversely, in the event of a range change downshift in the range group, there is a shift back from the S gear into the L reduction group and, in the main transmission, there is an upshift from the smallest gear into the largest gear. In deviation from this, with a range change shift, jumping over one gear inside the main transmission is also possible. Since almost simultaneous shifting of the main transmission and the range group is difficult to bring about manually, the range groups of known range group transmissions are usually shifted automatically using synchronized shifting clutches.
The chronological course of a range change shift can be differently configured corresponding to the respective goal. Usually with a range change shift following disengagement of the startup and shifting clutch, the previous gear of the main transmission is disengaged; the new gear of the range group is synchronized and subsequently engaged; the new gear of the main transmission is synchronized and subsequently engaged and finally the startup and shifting clutch is engaged again.
With this shifting sequence, which is also known from German Patent DE 197 54 726 A1, the moments of inertia to be synchronized of the respective input shafts and the transmission elements connected with these as well as the therewith necessary shifting forces are, in each case, minimized by a delay or an acceleration. But the disadvantage is that acceleration of the shifting sequence, especially within a range group, is possible only with a considerable expenditure in equipment. Thus a synchronizing apparatus must be provided for accelerating the shifting process in the range group, such as a transmission brake, to retard and an auxiliary drive to accelerate the input shaft of the range group which, however, only comes into question for special motor vehicles due to space and cost reasons.
With another shifting sequence in accordance with German Patents DE 100 36 511 A1 and DE 101 52 857 A1, it is provided that in each case prior to shifting the main transmission, the previous gear of the range group is disengaged and, after shifting the main transmission, the new gear of the range group is synchronized and engaged. In this way, the synchronization of the main transmission can, indeed, take place with the startup and shifting clutch engaged by a corresponding actuation of the drive motor. The synchronization of the downstream range group is, nonetheless in this way, disadvantageously made difficult and delayed due to the great rotational speed difference and the higher moments of inertia associated with the input shaft of the range group.
Against this background, the object underlying the present invention is to propose a shift control method for an automatic range group transmission of the type mentioned at the beginning with which a range change shift can be conducted more rapidly than previously possible without great expenditure in apparatus.